Plastic is harming seabirds even more than we realized

Plastic pollution in the ocean is an urgent crisis. While most people recognize its physical dangers, new research reveals deeper biological threats.

A new study shows that ingested plastic can release hormone-altering chemicals in northern fulmars, a species of seabird that inhabits the North Atlantic and North Pacific.

Researchers from UC Santa Cruz and the San Diego Zoo Wildlife Alliance conducted this study. Their findings suggest that plastics can chemically interfere with birds’ hormonal systems. This is more than a surface-level threat – this interference may affect survival, reproduction, and behavior.

What happens after seabirds ingest plastic?

The researchers analyzed estrogen receptor (ER) activity in northern fulmars. ERs are vital for reproduction and development. Many plastic-associated chemicals mimic or block estrogen, acting as xenoestrogens.

To investigate this, scientists compared human and fulmar ER responses using a luciferase-reporter assay.

Both species’ ERs were activated by common plastic additives like bisphenol A (BPA), bisphenol S (BPS), 4-octylphenol (4-OP), and benzyl butyl phthalate (BBP). They were inhibited by known toxicants such as tetrabromobisphenol A (TBBPA) and polychlorinated biphenyls (PCBs).

This similarity in response raises concerns about ecosystem-wide effects. The chemicals leached from plastics don’t just pass through animals – they bind to key receptors that control biological systems.

Plastic chemicals harm birds for days

Lead author Liesbeth Van Hassel emphasized that plastic ingestion may have deeper effects.

“We’ve long known that plastic ingestion can cause physical harm to seabirds, but this study shows it may also have hidden biological effects,” she said.

“What’s especially concerning is that these chemicals don’t just pass through – they interact with key hormone receptors in the body.”

In the study, plastic recovered from 27 fulmars’ stomachs was soaked in solvent. The leachates were tested on cloned fulmar hormone receptors. Thirteen of the birds had plastics that activated or blocked hormone receptors on day one.

Nine of those still showed disruption after five and fourteen days, revealing a prolonged threat.

Damage caused by plastic chemicals

The study also analyzed the polymers in the ingested plastics. Using infrared spectroscopy, they identified polyethylene (60%), polypropylene (35%), polyethylene terephthalate (3%), and 2% unidentifiable materials.

However, the type of plastic didn’t predict hormonal response. This suggests the additives – not the base materials – cause the damage.

“Some of these plastics kept leaching active chemicals for two weeks,” said Christopher Tubbs, co-author and Associate Director of Reproductive Sciences at San Diego Zoo Wildlife Alliance.

“This suggests seabirds are not only swallowing harmful materials — they may be getting a continuous dose of hormone-altering chemicals.”

Seabirds, humans, and plastic chemicals

While fulmars and humans showed similar reactions to certain chemicals, sensitivity varied by substance and concentration.

For instance, human ERs responded more strongly to BPA and BPS, while fulmar ERs were more sensitive to 4-OP. These differences highlight the importance of testing species-specific hormone systems rather than relying only on human models.

Previous research has shown that different bird species experience unique endocrine effects. Quail, for example, showed reproductive issues and lowered estrogen after ingesting plastic. Fulmars, with long retention times and a tendency to consume floating plastics, are especially vulnerable.

A global warning from seabirds

Northern fulmars are often used as indicators of ocean plastic pollution. Their surface-feeding habits expose them to microplastics, and their stomachs can retain these for weeks or months. This extended retention increases the chance of chemical absorption.

The team’s study offers the first data showing that a seabird’s estrogen receptors can be disrupted by chemicals released from actual ocean-recovered plastic.

It provides a cautionary example of how plastics are harming wildlife not just physically, but chemically and hormonally.

Uncovering plastic’s deeper risks

This research is part of a broader collaboration between the San Diego Zoo Wildlife Alliance and UC Santa Cruz.

The goal is to uncover hidden risks to species and apply this knowledge in conservation science. The team’s findings highlight the need to reduce plastic waste and limit the use of harmful additives.

Plastics may look inert, but inside a bird’s stomach, they become chemical hazards. This study reveals how ocean pollution is silently rewiring the biology of marine species – one swallowed fragment at a time.

The work was supported by Oikonos Ecosystem Knowledge, the NOAA North Pacific Observer Program, and the CDFW-OSPR Marine Wildlife Veterinary Care and Research Center. These partnerships enabled collection, lab work, and analysis vital to this study.​

The study is published in the journal Environmental Pollution.

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